Probably some character that has not been featured in any shorts or comics before. And, of course, the point of the shorts is to further explore the characters.
Dva, for example. I don't think'd be a lot of fun to just see her fight a huge robot, but would be interesting to see a justaposition of her life as a celebrity, staring on movies and comercials etc, and her life as a hero. Maybe show some depth to her character, that she is a little more than her persona of a shallow cutsy teenager. She could show some true devotion to her people and cause.

Zenyatta would also be great. It could show how exactly the Shimbali work and why Zenny decided to leave them. Maybe show his life in the temple, the moment where he decided to leave and end with his travels, him meeting people and teaching them(Genji included)

A more fleshed out Reaper cinematic that isn't just him being a baddie McVillian edgelord. Perhaps his backstory and the fallout of Overwatch/Blackwatch civil war and Mercy's procedure that keeps him between life and death.